Multiple input multiple output (MIMO) is an advanced antenna technique to improve the spectral efficiency and thereby boost the overall system capacity. MIMO techniques use a common notation (M×N) to represent MIMO configuration in terms number of transmit (M) and receive antennas (N). The common MIMO configurations used or currently standardized for various technologies are: (2×1), (1×2), (2×2), (4×2), (4×4), (8×2), (8×4) and (8×8). The configurations represented by (2×1) and (1×2) are special cases of MIMO that correspond to transmit diversity and receiver diversity respectively. Current LTE and HSPA standards (up to Rel.12) supports the use of a 1-dimensional array of co- or cross-polarized antenna ports. Under development in 3GPP is standard support for 2-dimensional antenna ports, where antenna ports are located in both vertical and horizontal dimensions.
Multiple antennas employed at the transmitter and receiver may significantly increase the system capacity. By transmitting independent symbol streams in the same frequency bandwidth, usually termed as spatial multiplexing (SM), a linear increase in data rates is achieved with the increased number of antennas. On the other hand, by using space-time codes at the transmitter, reliability of the detected symbols can be improved by exploiting transmit diversity. Both these schemes assume no channel knowledge at the transmitter. However, in practical wireless systems such as the 3rd generation partnership project (3GPP) long term evolution (LTE), HSDPA and WiMAX and systems, the channel knowledge can be made available at the transmitter via feedback from the receiver to the transmitter. A MIMO transmitter may utilize this channel information to improve system performance with the aid of precoding. In addition to beam forming gain, the use of precoding may avoid the problem of an ill-conditioned channel matrix.
In practice, complete channel state information (CSI) may be available for a communication system using the time division duplex (TDD) scheme by exploiting channel reciprocity. However, for a frequency division duplex (FDD) system, complete CSI is more difficult to obtain. In a FDD system, some CSI knowledge may be available at the transmitter via feedback from the receiver. These systems are called limited feedback systems. There are many implementations of limited feedback systems such as codebook based feedback and quantized channel feedback. 3GPP LTE, HSDPA and WiMax recommend codebook based feedback CSI for precoding. Examples of CSI are channel quality indicator (CQI), precoding index (PCI), precoding matrix indicator (PMI), and/or rank indicator (RI). One or a combination of different types of CSI may be used by the network node (e.g. NodeB in UTRAN or eNodeB in LTE) for one or more resource assignment related tasks such as scheduling data to UEs, rank adaptation of MIMO streams, precoder selection for MIMO streams, etc.
In codebook based precoding, a predefined codebook is defined both at the transmitter and the receiver. The entries of the codebook may be constructed using different methods such as, for example, Grassmannian, Lloyd's algorithm, DFT matrix, etc. The precoder matrix is often chosen to match the characteristics of the NR×NT MIMO channel matrix H, resulting in so-called channel dependent precoding. This is also commonly referred to as closed-loop precoding and essentially strives for focusing the transmit energy into a subspace which is strong in the sense of conveying much of the transmitted energy to the UE. Alternatively, it may be used to avoid transmitting energy in directions that would interfere with another UE. In addition, the precoder matrix may also be selected to strive for orthogonalizing the channel, meaning that after proper linear equalization at the UE, the inter-layer interference is reduced. At the receiver it may be common to find the SINR with different codebook entries and choose the rank and/or precoding index which gives highest spectral efficiency (capacity).
However, the network may choose to utilize only a small number of precoding elements for a variety of reasons and indicate these to the UE. This selection may be referred to as codebook subset restriction or precoding weight restriction. One such reason could be that some transmit directions should be avoided since there is a low power eNB (known as a small cell) placed in that direction, and which may be interfered if the eNB transmits towards that direction. With codebook subset restriction the UE may be restricted to only select and report precoders that will be used by the network.
Overview of Codebook Subset Restriction
According to 3GPP standard TS 36.213, a UE is restricted to report PMI and RI within a precoder codebook subset specified by a bitmap parameter codebookSubsetRestriction configured by higher layer signaling. For a specific precoder codebook and an associated transmission mode, the bitmap may specify all possible precoder codebook subsets from which the UE can assume that the eNB may be using when the UE is configured in the relevant transmission mode. Codebook subset restriction is supported for open-loop spatial multiplexing, closed-loop spatial multiplexing, multi-user MIMO and closed-loop Rank=1 precoding. The resulting number of bits for each transmission mode is given in Table 1. The bitmap forms the bit sequence aAc−1, . . . , a3, a2, a1, a0, where a0 is the LSB and aAc−1 is the MSB and where a bit value of zero indicates that the PMI and RI reporting is not allowed to correspond to precoder(s) associated with the bit. The associations of bits to precoders for the relevant transmission modes are given as follows:
TABLE 1Number of bits in codebook subset restriction bitmapfor applicable transmission modes.Number of bits Ac2 antenna4 antennaportsportsTransmission modeOpen-loop spatial24multiplexingClosed-loop spatial664multiplexingMulti-user MIMO416Closed-loop rank = 1416precoding
In HSPA:
According to 3GPP standard TS 25.214, a UE is restricted to report precoding control index (PCI), and number of transport blocks preferred (NTBP) within a precoder codebook subset specified by a bitmap parameter PrecodingWeightRestriction configured by higher layer signaling. The bitmap can specify all possible precoder codebook subsets from which the UE can assume the NodeB may be using when the UE is configured in MIMO mode with four transmit antennas. The bitmap forms the bit sequence a63, . . . , a3, a2, a1, a0 where a0 is the LSB and a63 is the MSB and where a bit value of zero indicates that the precoding indices reporting is not allowed in the NTBP/PCI/CQI report.
Active Array Antenna Systems:
Referring to FIG. 1, an active antenna array is illustrated. Active-array-antenna systems (AAS), where RF components, such as power amplifiers 102a . . . n and transceivers are integrated with an array of antennas elements 101a . . . n to boost baseband signals 103, as shown in FIG. 1, offer several benefits compared to traditional deployments with passive antennas connected to transceivers through feeder cables.
FIG. 2 illustrates a passive antenna array. FIG. 2 shows an example of passive antennas array system where the baseband signals 103 are boosted by power amplifier 202 and connected to the antennas 201a . . . n by longer feeder cables. By using active antenna array as in FIG. 1, not only are cable losses reduced, leading to improved performance and reduced energy consumption, but also installation may be simplified and the required equipment space may be reduced.
There are many applications of active antennas for example cell specific beamforming, user specific beamforming, vertical sectorization, massive MIMO, elevation beamforming etc. It may also be an enabler for further-advanced antenna concepts such as deploying large number of MIMO antenna elements at the eNodeB. For these reasons, 3GPP started a study item investigating the feasibility to increase the number of transmit antennas to 16/32/64 for various purposes and also extending the CSI feedback to support 2-dimensional antenna arrays where the up to 64 eNodeB antenna ports are distributed both in vertical and horizontal directions.
Coverage Enhancements in LTE
Work is ongoing in 3GPP to enhance the coverage for machine type communication (MTC) devices, a special category of UEs, and to achieve in the order of 15-20 dB coverage enhancements in LTE multiple physical channels and physical signals will need to be improved. Since the required improvements are large (20 dB coverage improvements may be equivalent to operation at 100 times lower signal-to-noise ratio) and LTE is already very good, (i.e. there is no known flaw in LTE that can provide improvements anywhere near 100 times) it is likely that plain old repetition will provide most of the required coverage improvements. Current LTE signals may not easily be repeated approximately 100 times, for example, due to timing constraints during connection setup and other procedures. Therefore, new signals may need to be defined for this purpose.
A likely outcome of the 3GPP work on enhanced coverage MTC devices is therefore support of single layer transmission, since multiple layer MIMO transmission is not applicable for UEs with extreme coverage extension due to poor SINR.
The approaches described in the Background section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in the Background section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in the Background section.